1. Field of the Invention
This invention relates to a printed wiring board, and particularly to a printed wiring board capable of being connected by blind holes which allow for high density mounting of components, and to a method of manufacturing the printed wiring board.
2. Description of the Prior Art
A conventional printed wiring board is described below with reference to FIGS. 3(A) and 3(B).
More specifically, a printed wiring board 100 includes a dielectric resin substrate 102, copper conductor pads (inner layer pattern) 104 and 109 on opposing surfaces of the substrate 102, dielectric resin layers 101 and 103 laminated on opposing surfaces of the substrate 102, outer layer copper reflow pads 108 and 113, and outer layer copper conductor lines 105 and 110. Blind holes 106 and 111 which have copper plating layers 107 and 112 are formed in the dielectric resin layers 101 and 103. Plated through holes 114 and 115 are formed through the layers 101 and 103 and the substrate 102 and have through hole copper plating layers 116 and 117 for connecting outer layers with the inner pads 104 and 109.
The reflow pads 108 and 113 which mount electric components on the surface are connected to the copper layers 107 and 112 of the blind holes 106 and 111 by narrow conductor lines 105 and 110.
The inner layer conductor pads 104 and 109 are connected to the narrow conductor lines 105 and 110, and thus the outer layers 108 and 113, by the copper plating layers 107 and 112 of the blind holes 106 and 111.
FIG. 4(A)-FIG. 4(G) illustrate a conventional printed wiring board manufacturing process which uses a laser beam.
In the first step of the process, the inner layer conductor pads 104 and 109 are etched and formed on both surfaces of the dielectric resin substrate 102 as shown in FIG. 4(A).
In the second step of the process, the dielectric resin layers 101 and 103, and copper foil layers (outer layers) 105a and 110a are laminated and pressed onto the substrate, as etched and formed in the first step shown in FIG. 4(A), in a heated environment.
In the third step of the process, special windows 118 and 119, through which a laser beam can be radiated, are etched as shown in FIG. 4(C).
In the fourth step of the process, laser beams 120 are radiated through the windows 118 and 119 to form blind holes 106 and 111 which reach the inner layer conductor pads 104 and 109, as shown in FIG. 4(D).
In the fifth step of the process, through holes 114 and 115 are drilled as shown in FIG. 4(E).
In the sixth step of the process, blind holes 106 and 111, and through holes 114 and 115 are plated with plating layers 107, 112, 116 and 117 as shown in FIG. 4(F) for connecting the copper foil layers 105a and 110a with the inner layer pads 104 and 109.
In the last step of the process, the copper foil layers 105a and 110a are etched to form narrow outer layer conductor lines 105 and 110 and reflow pads 108 and 113, as shown in FIG. 4(G).
However, as stated above, on the conventional printed wiring board 100, the reflow pads 108 and 113 are used only for mounting surface mount components, and the narrow conductor lines 105 and 110 are used only for connecting to the blind holes 106 and 111, respectively, because the conductor pads around the blind holes 106 and 111 are higher than the outer layer copper areas and flat surfaces are necessary for mounting the surface mount components.
Accordingly, the maximum mounting density for surface mounted components is relatively low.
Furthermore, high density wiring boards with many layers printed thereon are manufactured by repeating the above process, and the conductor pads (inner layer patterns) 104 and 109 are largely shifted from their specified positions due to material thermal expansion. When, in the laser process, the blind holes are formed in positions offset from the inner layer conductor pads, the conductive areas of the hole bottoms become smaller than the specified area.